Ceramic matrix composites (CMCs) have relatively low thermal conductivities and are thus well suited for use in high temperature environments for long periods of time. However, the low thermal conductivity poses a serious problem for use of CMC's in high temperature applications involving very rapid heat-up, such as the flowpath of a gas turbine engine. In applications such as turbine engines, the low thermal conductivity of CMCs causes high thermally-induced stresses in the component. Mathematically simplified, this relationship is represented by the equation σ=E α/K; where σ is the thermally-induced stress, E is the elastic modulus, α is the thermal expansion and K is the thermal conductivity.
The current method to reduce thermally induced stresses on CMCs in rapid heat-up environments is to incorporate cooling holes into the component. Current methods involve drilling of small cooling holes (˜0.060 in.) at regular spacing throughout the whole CMC component. This usually requires thousands of holes even for relatively small parts. Laser drilling and ultrasonic drilling are the industry standards for drilling the holes. While both of these processes work well, both are time consuming and expensive, especially for complex-shaped components. Complex-shape components must be repeatedly repositioned, or “fixtured” to expose the desired part surface and to set the appropriate angle for drilling of each hole.
Cooling provided by drilled holes is very discrete or localized within the body of the CMC. Cooling resulting from these holes essentially works by providing a film of cooler air over the surface of the component, thereby reducing the surface temperature of the CMC part, which ultimately reduces the thermal gradient through the CMC. Holes may be blind holes (i.e., not perforating through the entire CMC and not connected to other holes or channels) through holes (perforated through the CMC but not connected to other cooling holes or channels) or (i.e., networked holer connected to other cooling holes or channels). Preferably, at least some of the holes are through holes to ensure proper film cooling.
Therefore, the need exists for a process wherein adequate internal cooling is promoted within the CMC part, and which is significantly less expensive than the current drilling processes.